Chasing down a miss

[Jdoggat22]

I didnt see anywhere anything about the MAF. Clean the MAF and check the temp sensor on the front of the thermostat housing, or just replace it. That temp sensor sends the ecm engine temp, if it runs good while cold and bad while hot. The ecm could be running the wrong parameters for that temp. The fisrt thing i always do is clean the MAF, a dirty or bad MAF can do some really wierd stuff and not even throw a code for anything.

Cheap and quick options there.

 

[Erik the Awful]

Just to clarify, a noid light will only tell you if the computer's trying to fire an injector. It tells you nothing about the condition of the injectors.

I pull the injector plugs one cylinder at a time until I figure out which cylinder doesn't make a difference. About a month ago my son's Crown Vic started dead-missing while I was driving it to his place. I pulled into O'Reilly's, checked and found an injector plug broken, bought a new plug and a pair of crimpers, and installed it right there in the parking lot. We did the same thing to figure out which ignition coils were dead on his previous Crown Vic.

 

[8T7K5]

I didnt see anywhere anything about the MAF. Clean the MAF and check the temp sensor on the front of the thermostat housing, or just replace it. That temp sensor sends the ecm engine temp, if it runs good while cold and bad while hot. The ecm could be running the wrong parameters for that temp. The fisrt thing i always do is clean the MAF, a dirty or bad MAF can do some really wierd stuff and not even throw a code for anything.

Cheap and quick options there.

But would either of those cause the misfire on specific cylinders or would it just be random? It's always #3 constant, #4 intermittent.

 

[8T7K5]

Just to clarify, a noid light will only tell you if the computer's trying to fire an injector. It tells you nothing about the condition of the injectors.

I pull the injector plugs one cylinder at a time until I figure out which cylinder doesn't make a difference. About a month ago my son's Crown Vic started dead-missing while I was driving it to his place. I pulled into O'Reilly's, checked and found an injector plug broken, bought a new plug and a pair of crimpers, and installed it right there in the parking lot. We did the same thing to figure out which ignition coils were dead on his previous Crown Vic.

I ordered a noid light set and an injector tester from Amazon. Also gonna be getting an inline spark tester to see if those two cylinders are getting fire. The issue with the big block is that it's impossible to pull an injector and crank the engine. I credit GM and their idiotic dual intake setup for this. As was said in a previous reply I'll have to pull the upper intake, pull those two injector plugs, run wires out the front of the intake to test them. Very time consuming to pull the intake to check each one individually. Makes me want to ditch it for a Sniper or do an 8.1 swap.

 

[Pinger]

I've asked this in another thread, looking for a little more specific info if possible. My 99 Vortec big block has an intermittent miss on #4 and constant miss on #3. Injectors are new Bosch, regulator is new, distributor, cap, rotor, plugs, wires are all new. Upper/lower intake gaskets are new. Fuel pressure and compression are all good. I'm at my wit's end and way beyond my abilities to diagnose this. It seems to run ok when it's cold but once it warms up it gets worse. I've ordered a fuel injector tester to check the injectors on 3 and 4. The injectors came from ebay and I suspect that maybe they're Chinese copies, if those two are the problem. I've contemplated a Sniper but ideally would rather the factory system work like it should because if the injectors aren't the problem I've fixed nothing.

That it worsens when warm suggests over-fuelling if the problem is with the fuel system.
Once it's up to temp, do plugs from 3 and 4 cylinders show signs of over-fuelling? If so, stuck open injectors are possibly the cause (less likely on new unless faulty from the factory). If there are stuck open injectors I'd expect fuel pressure (after prime without engine running) to dissipate quickly.

 

[8T7K5]

That it worsens when warm suggests over-fuelling if the problem is with the fuel system.
Once it's up to temp, do plugs from 3 and 4 cylinders show signs of over-fuelling? If so, stuck open injectors are possibly the cause (less likely on new unless faulty from the factory). If there are stuck open injectors I'd expect fuel pressure (after prime without engine running) to dissipate quickly.

Haven't checked the plugs yet but I will. I did notice that after replacing all 8 injectors and the regulator I still smelled gas from under the intake, just like before I replaced them. I'm hoping it's just 2 bad injectors. If that's the case they'll be replaced with the real ones from 5 O motorsports.

 

[Pinger]

Haven't checked the plugs yet but I will. I did notice that after replacing all 8 injectors and the regulator I still smelled gas from under the intake, just like before I replaced them. I'm hoping it's just 2 bad injectors. If that's the case they'll be replaced with the real ones from 5 O motorsports.

If - and it is just an if - it is that, gasoline could be reaching the crankcase (an ignition misfire could do that too though). Any smell of gasoline in the oil?

 

[8T7K5]

Finally got around to testing the injectors on #3 and #4. Pulsed them and both are working fine. Tested resistance and voltage on the harness, both are fine. What's next since plugs, wires, cap, rotor, and distributor are all new?

 

[Schurkey]

As said, testing the ignition is easier than testing the fuel system.

I would warm-up the engine so that it's misfiring. Put a spark-tester calibrated for HEI ignition on those two plug wires, idle the engine, see if you get a consistent spark at each of the two plug wires.

Good, reliable spark? Faulty spark plugs, or not an ignition problem.

No, or erratic spark? Check other plug wires. If they're good, you have a bad cap or defective wires.

 

[8T7K5]

As said, testing the ignition is easier than testing the fuel system.

I would warm-up the engine so that it's misfiring. Put a spark-tester calibrated for HEI ignition on those two plug wires, idle the engine, see if you get a consistent spark at each of the two plug wires.

Good, reliable spark? Faulty spark plugs, or not an ignition problem.

No, or erratic spark? Check other plug wires. If they're good, you have a bad cap or defective wires.

Ignition is my next step once I get it back together. If it's not ignition, knowing it has good compression, I'm not sure where to go from there.